Music synthesizers are, of course, known in the art and generally comprise a keyboard provided with a multiplicity of note-selection keys whose switches are connected to various oscillator circuits, filters or like electronic controls, the outputs of which can be combined to form electrically generated tones, chords and voices in the production of music.
Various expression devices are known independently or have been associated with such synthesizers. The term "expression" is here used to refer to a modulation applied to the pure tone or to the voices generated by the instrument and may include, for example, a tremolo or the like which is superimposed upon the output.
In the music synthesizer field it has become desirable to provide a number of expression modes, for example, under the control of a foot pedal, or presenting the velocity as the key is depressed, or generated or controlled by a joystick or other implement.
Of considerable interest is an expression mode which modifies the output as a function of the pressure applied to the keyboard.
Most of the devices provided for this purpose are true pressure-electronic transducers in which an element exposed to pressure and coupled to the keyboard is provided to generate an electrical output which is used to modulate the signal resulting from the operation of the keys. Other systems can be conceived of for this purpose, however, which respond not directly to pressure, but indirectly to pressure because they are activated by movement of the keyboard against, for example, the force of a restoring means such as a spring.
Differential transformers, Hall effect systems and the like have been proposed for this purpose, but have not been successful previously because on the one hand they tend to be too expensive or complex to be readily incorporated into keyboards for music synthesizers of the type described and, on the other hand, tend to be unstable and unreliable, especially with long term use.